Boobie Grin Share definition Flag (noun) the crease created between two large breasts when they're pressed together in the middle. The valley of the cleavage. With that push-up on, I *swear* Donna's got four inches of boobie grin goin' on. Boobie Grin by Holly M S May 24, 2010 0 0 Get merch Get the Boobie Grin mug. Buy Now — $32.95 See more merch